Removal of iodine from nitric acid solutions

ABSTRACT

D R A W I N G A METHOD FOR REMOVING RADIOACTIVE IODINE FROM NITRIC ACID SOLUTION BY ISOTOPICALLY DILUTING THE IODINE AND AUTOCLAVING THE SOLUTION PRIOR TO SEPARATION.

Feb. 12, 1974 G, CATHERS ET AL 3,792,154

REMOVAL OF IODINE FROM NITRIC ACID SOLUTIONS Filed March 6, 1972 (I HU 1 NOIlVHlNI-IONOO I United States Patent Office 3,792,154 Patented Feb. 12, 1974 U.S. Cl. 423-390 6 Claims ABSTRACT OF THE DISCLOSURE A method for removing radioactive iodine from nitric acid solution by isotopically diluting the iodine and autoclaving the solution prior to separation.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the U.S. Atomic Energy Commission.

This invention relates to the art of removing radioactive iodine from nitric acid solutions.

In reprocessing fuels from a liquid metal fast breeder reactor (LMFBR) it is desirable to dissolve the entire fuel element in nitric acid and to then separate the desired radioactive elements one at a time from the solution. The principal purpose of the reprocessing is to recover uranium and plutonium for re-use in another fuel element. One of the radioactive elements in soltuion is iodine. Radioactive iodine is comprised of a major amount of iodine-131 with a small amount of iodine-129. The proportions of the two isotopes vary with time in accordance with their half lives. In order to protect the environment and operating personnel from possible exposure to radioactive iodine it is desirable to separate and isolate the iodine isotopes from the solution prior to further processing. Radioactive iodine is particularly dangerous because of its volatility, and its ability to be carried by vegetation and transmitted through milk for human ingestion. Once ingested by a human being the iodine concentrates in the thyroid where it can cause severe cell damage and cancer.

Prior to the present invention, radioactive elements have been removed from solution by an appropriate separation process -until the concentration of the radioactive element becomes so low that further separation is impractical. At this point the radioactive isotope is diluted with a non-radioactive carrier isotope preferably of the same element, and the separation process carried out again. The concentration of the radioactive element in the second separated portion is usually equal to the concentration of the radioactive isotope in the solution relative to the concentration of the non-radioactive isotope in the solution. The merits of a radioactive decontamination process are usually measured by the decontamination factor, i.e., original activity divided by the final activity. The above prior art process is called isotopic dilution and very high decontamination factors can usually be achieved by its practice.

SUMMARY OF THE INVENTION In attempting to use the prior art isotopic dilution process in the separation of radioactive iodine from nitric acid solution, it was unexpectedly discovered that this process was ineffective for removing radioactive iodine at molarities of less than about Reprocessing safety requirements necessitate the removal of radioactive iodine to less than about 10- molarity for fuels aged less than 30 days and to less than 10- molarity for fuels aged more than 30 days.

It is thus an object of this invention to provide a process whereby radioactive iodine may be effectively and efliciently removed from nitric acid solutions to concentrations of less than l0- to 10- molarity.

This object as well as other objects is accomplished according to this invention by separating radioactive iodine from nitric acid solution followed by isotopic dilution, autoclaving and further separation to achieve high decontamination factors.

BRIEF DESCRIPTION OF THE DRAWING The single figure of drawing represents a plot of percent radioactive iodine remaining versus percent of initial volume distilled for various heat treatments according to this invention.

DETAILED DESCRIPTION According to this invention it was discovered that conventional isotopic dilution techniques, when applied to the removal of radioactive iodine from nitric acid solution, did not produce a significant increase in the decontamination factor. Attempts to achieve further separation using the conventional process resulted in the removal of the diluent isotope but not the radioactive isotope. However, it has additionally been discovered that significant further separation is achieved by autoclaving an isotopically diluted solution prior to performing additional separation.

The exact reason for this phenomena is not entirely understood. However, it is postulated that a small amount of the radioactive iodine forms a non-distillable and unidentified complex with the nitric acid. For the sake of convenience this complex for the 1 isotope can be represents as H]. The brackets, [1, represent the species which comes from the nitric acid solution. When nonradioactive iodine, i.e., 1, is added to the solution, the kinetics of the exchange between the 1 in the complex and the 1 is such that no appreciable exchange is observed. When the solution is heated under pressure in the autoclave the kinetics of the exchange process are considerably increased such that the following isotopic exchange reaction occurs:

The complex formed in the solution is postulated to be an oxidized form of iodine. Since the iodine must be in an essentially free state, i.e., I to distill from the solution, this oxidized form of iodine would prevent such distillation. It has been further found that sparging the solution with ozone will also increase isotopic exchange, thus supporting the postulation that the iodine is in an oxidized state. The ozone apparently oxidizes the iodine diluent thus making conditions more favorable for isotopic exchange. Treating the radioactive iodine nitric acid solution with a reducing agent prior to separation has also been found to increase the decontamination factor of the first separation. The reducing agent, a nitrate, such as sodium nitrite, seems to reduce the oxidized iodine back to distillable free iodine. The above measures which support the oxidized iodine postulation may be used in addition to or independently of the processof this invention to achieve iodine separation; however, the process of this invention is presently the best known independent process for decontaminating a nitric acid radioactive iodine solution.

The preferred embodiment of this invention comprises carrying out a first distillation of about 10 to 20 and preferably about 15 percent of the volume of a 3-6 molar aqueous solution of nitric acid. Under actual plant conditions this original solution contains a radioactive iodine molarity of about 10*. After the first distillation, the solution is isotopically diluted with carrier iodine,

preferably by adding potassium iodide which readily oxidizes to I For the sake of convenience, the carrier iodine is usually added in an amount such that the original iodine concentration is restored. The amount of iodine added, however, must be suflicient to provide a distillable level of iodine, i.e., an iodine molarity of at least 2.5 X 10 The autoclave treatment of this invention may be carried out in a batch operation; however, a continuous process is preferred. In a continuous process an autoclave of fixed volume is used. The residence time in such an autoclave is measured by dividing the fixed volume by the flow rate. It is further preferred to use an autoclave constructed of glass rather than metal. A glass autoclave, for reasons unknown, has been found to give results which are superior to those achieved in a metallic autoclave. However, both types of autoclaves are operable in the process of this invention.

Varied autoclaving residence times and temperatures may be used in the process of this invention depending upon the desired results. However, autoclaving temperatures of at least 120 C. are essential for producing any significant results. For the purposes of this invention, the most efiicient process has been found to include an autoclave residence time of from 25 to 50 minutes at a temperature between about 150 to 180 C.

Any conventional distillation technique may be used after the autoclave treatment. It is preferred to bubble N gas through the solution during distillation. This step is not essential to the process, but improved results are achieved by its practice. The iodine-water distillate is collected by condensation at ambient temperature in an acid corrosion resistant container such as glass or stainless steel.

The following specific examples are given as illustrations of the process of this invention with reference to the accompanying figure of drawing.

A four molar aqueous solution of nitric acid containing iodine-131 at tracer level was prepared by adding 1 milliliter of a 10 molar sodium bisulfite solution con- 'taining iodine-131 as sodium iodide at about 10- molarity to 300 milliliters of the nitric acid solution. The I was then fixed by boiling the solution at about 108 C. in a flask equipped with a reflux condenser. This assured that the non-distillable complex had formed. The solution now had a radioactive iodine molarity of about 10- at an activity of 1.0 millicurie. Carrier iodine was then added to the solution in the form of potassium iodide to give a 2.5 10- molar iodine solution. Seven 300-milliliter samples of this solution were prepared. Six of the samples were autoclaved at various temperatures for varying periods of residence time in a continuous autoclave constructed of Zircaloy-Z. Zircaloy-Z is a zirconium base alloy containing about 1 wt. percent tin, 0.1 wt. percent iron, 0.05 wt. percent nickel, and 0.1 wt. percent chromium. The remaining sample served as a control sample and was not subjected to an autoclave treatment. The following chart shows the sample number as corresponding to the numbered curves of the drawing and the heat treatment received by the samples.

Each of the samples was distilled in a three-neck flask with a heated off-gas tube at a distillate collection rate of l-milliliter per minute while passing N 0 gas through the solution at a rate of 10 milliliters per minute. The amount of iodine-131 remaining in the solution was measured by the remaining activity after distilling 10, 20, and 30 percent of the initial solution volume. The effect of the autoclave treatment is vividly demonstrated by the graph of results shown in the figure of drawing.

It can be readily seen that by operating at higher temperatures and/or longer residence times more superior results are achieved. The process of this invention may also be repeated several times to achieve higher decontaminations.

What is claimed is:

1. A method for removing radioactive iodine from a nitric acid solution comprising the steps of isotopically diluting said solution with non-radioactive iodine, autoclaving said solution at a temperature of at least C. and distilling radioactive iodine from said solution to a concentration of less than about 10 molarity.

2. The method accordance to claim 1 wherein prior to said step of diluting, radioactive iodine is separated from said solution by distillation.

3. The method according to claim 1 wherein said nitric acid solution is a 4 to 6 molar aqueous solution.

4. The method according to claim 1 wherein said iodine is present in said solution at a molarity of at least 2.5 X 10 5. The method according to claim 1 wherein said autoclaving is carried out for a period of from 25 to 50 minutes at a temperature between and C.

6. The method according to claim 1 comprising the further step of bubbling N 0 gas through said solution during said distillation process.

References Cited UNITED STATES PATENTS 3/1944 Beekhuis, Jr. 423390 9/1962 Huising et al. 423500 U.S. C.l. X.R. 

